The present invention pertains to the field of processes for prefatory anti-forgery protection and consecutive machine-assisted authentication of genuine documents and other values. More particularly, the invention pertains to the use of processes for authenticating security items with Raman spectroscopy. In particular, the present invention relates to improved Raman-active compounds and compositions thereof. In some embodiments the present invention relates to use of improved Raman-active compounds on documents and other security items in the form of visible, camouflaged or completely concealed prints and markings for the purposes of authentication. Further, the present invention relates to techniques of composing and placing security markings on the items which need to be protected against forgery or counterfeiting.
Being directed, in some embodiments at least, to more reliable, facilitated, rapid and cost-effective mass authentication of genuine items xe2x80x9cin the fieldxe2x80x9d by virtue of machine-assisted recognition of the corresponding Raman-active molecular codes, tags and markings, the present invention relates also to employment of rugged, portable and affordable Raman spectrometers comprising: (a) corresponding types of radiation sources (such as lasers, light emitting diodes, etc.); (b) solid state radiation detectors of the corresponding type (such as CCDs, photo-diodes, detector arrays, etc.); (c) dispersive, electrooptic or interferometric means and filters for processing electromagnetic radiation (light) in the corresponding region of electromagnetic spectrum within the spectrometer; (d) optionally corresponding fiber optic conduits and probes for transporting the probing and/or the gathered (selectively transmitted, absorbed, reflected, scattered or emitted) light from or to the spectrometer; (e) corresponding computer and hardware responsible for analog to digital conversion of the signals; and (f) corresponding software responsible for processing, optimization, search and comparison of the spectral data as well as for presenting final results of the molecular code identification to an operator in an unambiguous and convenient form.
Other embodiments of the present invention are directed to a process which comprises (a) applying to a genuine item to be protected against duplicating, forgery or counterfeiting a marking material comprising an efficient Raman-active compound (or a composition of several such compounds) which, when irradiated with monochromatic radiation from the near infrared region of electromagnetic spectrum, is capable of emitting a detectable Raman spectrum, thereby forming a molecular code or security mark on the genuine item; (b) irradiating the security mark on the such protected genuine item with monochromatic radiation from the near infrared region of electromagnetic spectrum; (c) measuring the Raman spectrum of radiation scattered from the security mark when the mark is irradiated with monochromatic radiation belonging to the near infrared region of electromagnetic spectrum; and (d) processing the spectral data so obtained with the aid of corresponding dedicated software and presenting the results of the molecular code recognition to an operator in an unambiguous and convenient form.
Valuable and genuine items such as checks, passports, tickets, banknotes, licenses, identification cards and branded articles need to be produced in a manner which allows the genuine item to be reliably authenticated. For instance, in printing of paper values, highly diversified measures have been adopted to provide this, ranging from printing of easily recognizable visible features through printing discrete camouflaged visible features up to applying completely hidden from the human eye features which are only identifiable and may be verified by a machine.
The use of the human eye recognizable or camouflaged security features has been described, for example, in U.S. Pat. Nos. 5,630,869 and 5,807,625 disclosing reversibly photochromic printing inks; in U.S. Pat. Nos. 5,591,255 and 5,997,849 disclosing thermochromic printing inks; in U.S. Pat. Nos. 5,826,916, 5,772,248, 5,636,874, 5,873,604, 5,704, 5,449,200, 5,465,301, etc. Currently, a security printer is able to select from a variety of measures to prevent counterfeiting or forgery and to allow authentication. Any one document can include a range of such measures, and the choice of those that are actually included in a specific document already presents a formidable task to a wrongdoer. Nonetheless, there is a constant need to add, diversify and improve the measures that are employed, particularly those which lend themselves to present day security printing manufacturing, identification and sorting equipment.
Although the human visual system is highly capable of the perception of spectral energy within the visible region of electromagnetic spectrum, manifesting itself as brightness and color, it is less capable when used for quantification and recall. To perform these functions with much higher confidence, reproducibility, precision and speed than the human eye can do, spectral radiometers and/or spectrophotometers (spectrometers) can be used. Since an advanced machine-readable security feature can reveal its specific physical characteristics or attributes in different regions of electromagnetic spectrum (e.g., in the so called ultra-violet (UV), near-infrared (NIR) and middle-infrared (MIR) regions of the spectrum), the identification process should be performed with the aid of an appropriate spectroscopy method. Currently, it is possible to select, e.g., from the so called UV-, VIS-, Fluorescence, NIR-, MIR- and Raman spectroscopies which have been successfully used for many years for identification of chemical compounds in research, analytical and forensic laboratories. UV-, VIS-, NIR- and MIR-absorption spectra arise when respectively the ultraviolet (0.1-0.35 xcexcm), visible (0.35-0.75 xcexcm), near infrared (0.75-2.0 xcexcm) or middle-infrared (2.0-30.0 xcexcm) light incident upon a sample of the material is absorbed. Normal fluorescence can be detected by illuminating a material with light of an appropriate excitation wavelength (usually 200-600 nm) and by subsequent detection of the resultant spectral emissions with the aid of an electro-optical sensor. Raman spectra arise as a result of inelastic scattering of a laser light (which may have any wavelength from 0.25-2.0 xcexcm region) incident upon a sample of a material. The Raman effect occurs when small portions of photons in a laser light beam gain or loose some discrete amounts of energy upon colliding with vibrating molecules of a substance. This energy exchange alters the wavelength of the incident (exciting) light and the new wavelengths that are emitted constitute the Raman spectrum.
In any case, needed for such analyses spectrophotometers and/or radiometers perform the same type of physical measurement. Actually they calculate a series of weighted integrations of electromagnetic energy over wavelength. Spectrophotometers and spectral radiometers are generally capable of reporting tens to thousands of weighted integrations. The weighting functions for these instruments each usually have a common shape which is ideally a narrow triangle. For radiometers the sample is a radiant source, and the weighting function is the product of the spectral intensity of the source, a filter parameters, and the spectral sensitivity of the detector. Spectrophotometers generally use broad-band light sources and detectors, and the weighting function is provided by multiple filters, by monochromators, by spectrographs, or by interference techniques. Both spectral radiometers and spectrophotometers are intended to allow measuring transmittance (absorbance), reflectance or intensity spectrum of the sample with reference to wavelengths to be represented as a set of points, or a curve, or a spectrum.
More specifically, instrumentally measured UV- and, VIS-absorption (which, depending upon the sampling and the optical experiment geometry, may be measured in transmittance, absorbance or reflectance units) occurs when the wavelength of the incident probing beam of light turns out to be equal (that is in resonance with) to that of an optical absorption band in the material. The electrons responsible for the absorption are located on a number of molecular orbitals on subset of atoms in the compound, known as the chromophore. By absorbing an appropriate photon of the probing light, electrons temporarily transit from orbitals with lower energies to those having higher energies giving rise to the so called electron absorption bands. Usually, these phenomena involve a large number of different molecular orbitals in a compound and, as a consequence, UV- and VIS-absorption spectra of complex compounds are represented by a number of overlapping broad bands. While these absorption bands frequently have very high intensity (which is the measure of probability of a given electron transition) facilitating detectability of a compound, broadness of the absorption bands puts some restrictions on the analytical value of the method because very strict machine readability criteria (high optical resolution, high signal to noise ratio, narrow acceptability interval, high photometric accuracy, etc.) should be applied, especially when exploiting complex mixtures of several compounds as security features.
Conventional Middle-Infrared (MIR) absorption spectroscopy, in principle, has much higher analytical value compared to UV-VIS-absorption spectroscopy. Very roughly, MIR region is commonly defined as electromagnetic radiation with frequencies between 5,000 and 500 cmxe2x88x921 (2.0-20.0 xcexcm). When a normal molecular motion such as a vibration, rotation or lattice mode (as well as combination, difference, or overtone of these normal vibrations) results in a change in the molecule""s dipole moment, a molecule can absorb infrared radiation in this region of the electromagnetic spectrum. In other words, very selective resonant absorptions of monochromatic constituents of the infrared light from a broadband source by those molecular fragments of a compound which oscillate with the frequencies corresponding to the frequencies of the incident light are responsible for arising the infrared absorption bands. The corresponding frequencies and intensities of these infrared bands, the infrared spectrum, are then used to characterize the material. In terms of developing new anti-counterfeiting measures, the prime importance is the fact that infrared spectral information may be used to identify the presence and amount of a particular compound in a mixture. Modem instrumentation allows the collection of infrared spectra of materials at low-picogram levels. The ability of infrared spectroscopy to examine and identify materials under a wide variety of conditions has earned this technique the premier position as the xe2x80x9cworkhorsexe2x80x9d of analytical science. However, the main problem that always exists during analytical work in the infrared (and which always must be circumvented when applying IR-spectroscopy for security purposes also) is the so called xe2x80x9ctransparency window problemxe2x80x9d. Though the problem exists during absorption spectroscopy measurements in different spectral regions, it is especially severe in the infrared. Actually, despite the attractiveness of infrared spectroscopy for authentication purposes, this method yet did not find wide use xe2x80x9cin the fieldxe2x80x9d security measurements. The main obstacles for this are comparative sophistication and fragility of the instrumentation as well as the costs of the latter. xe2x80x9cTransparency window problemxe2x80x9d and labor costs for usually tedious and time consuming sample preparation of a sample before the analysis may be the additional reasons. Of importance, however, is the fact that MIR absorption spectroscopy, in principle, provides information which is similar to that obtainable by normal Raman scattering spectroscopy. Both infrared light absorption and Raman light scattering phenomena result in obtaining vibrational spectra of a compound. Both these spectra are mutually superimposable, but do not constitute replicas of each other. Rather, they are luckily complement each other giving a complete vibrational spectrum of a compound. While any one (MIR or Raman) vibrational spectrum of the two is a fingerprint of a compound and may be used for identification of the latter, the sum of the two spectra gives a much more detailed fingerprint of a compound and permits considerably more confident identification to be made. Moreover, as a rule of thumb, those normal vibrations which are Raman-active are considerably less active in the infrared absorption spectroscopy. And vice versa. It means that to facilitate identification of an unknown (or authentication of a searched, but known beforehand) compound, a judicious choice of a method is desirable.
NIR-absorption spectroscopy is very similar to the conventional infrared (MIR) analogue. Among advantages of this method are comparative simplicity of the necessary instrumentation and a possibility to run NIR-absorption spectra without any sample preparation. An essential drawback of the method, however, is that in NIR region of electromagnetic spectrum one has to exploit either overtones and/or combinations of the fundamental vibrations taking place in the MIR-region. In principle, the spectral bands corresponding to overtones and combinations have much lower intensity and are characteristically broadenedxe2x80x94the circumstances which worsen the analytical value of the NIR-absorption spectroscopy in terms of reliable identification of fakes and forgery.
In contrast to the above methods, Fluorescence spectroscopy (or fluorometry) is an emission technique. Normally it is based on use of fluorescent compounds (fluorophores) which typically are organic dye molecules. These molecules are capable of absorbing electromagnetic energy in a particular absorption wavelength spectrum and subsequently to emit light at more longer wavelengths. Usually, the sensitivity and reliability of detection of a fluorophore is related to both the type of the compound employed and the quality and type of equipment available to detect it. A wide variety of fluorescent dyes are available and offer a selection of excitation and emission spectra. And, in principle, it is possible to select fluorophores having emission spectra that are sufficiently different so as to permit their multitarget detection and discrimination. Unfortunately, detection methods which employ fluorescent labels are of limited sensitivity and reliability for a variety of reasons. Most importantly, with conventional fluorophores it is difficult to discriminate specific fluorescent signals from nonspecific background signals. Very often, the general characteristics of organic dye fluorescence are also applicable to background signals which are attributed to other compounds (e.g., vehicles, ink pigments, paper whiteners, etc). An extra problem with organic dye fluorophores is their photolytic decomposition (i.e., photobleaching). Due to this, even in situations where background noise is relatively low, it is often not possible to integrate a weak fluorescent signal over a needed detection time, since the dye molecules decompose as a function of incident irradiation dose absorbed in the UV, near-UV or even VIS bands.
Recently, a variety of inorganic phosphors with excitation in the infrared region have been introduced in to the market. These materials have seemingly greater potential in security applications, especially in areas where high-speed machine readability is required. Materials which can be excited with infrared radiation to emit radiation of a higher energy in the near infrared or visible region of the spectrum are known as xe2x80x9canti-Stokesxe2x80x9d or xe2x80x9cup-convertingxe2x80x9d phosphors. The xe2x80x9cnormalxe2x80x9d fluorescence is a quantum phenomenon in which energy is resonantly absorbed by the phosphor in the form of single photons of ultra-violet or visible radiation and, after energy losses for non-radiative phenomena, is emitted as photons of a longer wavelength light. In all normal fluorescence it follows that the exciting radiation must have a shorter wavelength than the emitted light. This observation is referred to as Stoke""s Law. In difference to this, the class of anti-Stokes phosphors has the ability to absorb two or three photons of longer wavelength infrared light and combine their energies to emit a single photon of, e.g. visible light. Phosphors of this class have characteristic excitation spectra in the infrared and near infrared, often coinciding with the wavelengths produced by NIR and IR light emitting diodes (LEDs) or lasers. An essential advantage of such anti-Stokes phosphors is that they are not susceptible to photobleaching and, due to the fact that excitation is performed in far red, NIR or IR region, they do not induce unwanted autofluorescence of substrates and impurities.
However, despite the great amount of efforts spent up to date to improve reliability of fluorescence spectroscopy, this method still possess a number of intrinsic drawbacks. Thus, for instance, when trying a xe2x80x9cblankingxe2x80x9d or xe2x80x9czeroingxe2x80x9d instrument, one usually expects to obtain a result where whatever residual signal of the blank material it will be eliminated or canceled out. Unfortunately, with fluorescence instruments this is never the case. The very name xe2x80x9cfluorescence blankxe2x80x9d is misleading, because even non-fluorescent materials can give rise to artifactual peaks. The problem arises from several sources. Artifactual peaks are generated in non-fluorescent blanks from (1) Rayleigh-Tyndall scatter, (2) Raman scatter, and (3) complex, but predictable, harmonic order reflections of these same scatter peaks caused by the diffraction grating employed in the monochromator or spectrograph of the instrument. Scatter emissions are generated by any material containing small particles: i.e., solids, liquids and gases. Because of the ubiquitous nature of the scattering phenomenon, artifactual peaks are generated by virtually any sample or material. Thus, there exist a significant need in the art for detection methods which permit sensitive optical and/or spectroscopic identification of specific label signal(s) with essentially total rejection of nonspecific background noise and which are nondestructive for different kinds of substrates carrying the labels.
In Raman spectroscopy, the scattered light includes light of the laser wavelength plus, at much lower intensity, light of additional wavelengths which are characteristic of the compound. Raman scattering effect is extremely weak; typically a few Raman (inelastically) scattered photons exist among millions of elastically scattered photons. The additional light appears at frequencies which are shifted from that of the laser beam by amounts equal to the frequencies of normal vibrations of the atoms in the compound. These frequencies are determined by the masses of the atoms comprising the material and the forces which hold them together. All molecules vibrate and do so in multitudinous fundamental modes, giving rise to changes in dipole moment (if nonsymmetrical) and/or polarizability (the ease with which electrons can be induced to respond to a potential gradient). The dipole change enables one to measure the frequency of molecular vibrations by infrared absorption; the polarizability change provides the driving force for Raman scattering. As fundamental vibrational modes are almost always unique for every chemical compound, the MIR- and Raman spectra are often used as its fingerprint. Raman spectroscopy has experimental advantages over infrared absorption. Firstly, window problems hardly exist, if visible or near infrared lasers are used as excitation sources. Secondly, since transmission through the sample is not a necessary prerequisite, hardly any sample preparation is required in Raman spectroscopy. Since Raman spectra reveal unique features that are specific to the materials under examination, this method provides an ideal way of detecting compositional variations in substances. In this way, the compound may be identified in various conditions, for example as a crystal, in solution, as a powder and in mixtures with other compounds. Thus, Raman spectroscopy might be rendered as, perhaps, the most promising candidate for providing highest degrees of protection against forgery and counterfeiting.
In the prior art, several examples of employing Raman spectroscopy for security purposes have been disclosed. Thus, for instance, use of polydiacetylenes as Raman-active components of an ink for printing security documents which then could be identified by Resonance Raman Spectroscopy (RRS) has been described in U.S. Pat. No. 5,324,567 (Ink compositions and components thereof, 1994, Bratchley et al., Thomas de la Rue and Company). In accordance with the teachings of this document, fourteen polydiacetylenes are suggested to be used in the form of particles having a maximum dimension of 40 xcexcm. The authors rely upon Resonance Raman Scattering which occurs when the wavelength of the incident laser beam is equal to, i.e. in resonance with, that of an optical absorption band of the material. They primarily rely on the circumstance that, under resonance conditions, the Raman scattered light should be greatly enhanced in intensity. According to their evaluations, the intensity of Raman lines due to collective vibrations of the backbone atoms of a polydiacetylene under resonance conditions can be at least 104 times greater than those arising from atomic vibrations in the side groups. Although the authors note that, in principle, the exciting laser light may be in the ultraviolet, visible or near infra-red regions of the spectrum, in their examples they used only a HeNe laser, emitting at a wavelength of 632.8 nm, and deliberately chosen so that the laser wavelength would fall within the optical absorption band of the material. Further, they exposed their samples to laser light and then measured with a photomultiplier a resonance Raman scattering intensity above the background fluorescence. Not providing factual Raman spectra of inks obtainable in accordance with their invention, the authors suggest to use only the five most prominent resonantly enhanced lines typical for RRS spectra of polydiacetylenes: one line at about 2100 cmxe2x88x921, two near 1500 cmxe2x88x921 and two between 1300 and 900 cmxe2x88x921, displaced to lower energy from the incident laser source wavenumber. The authors suggest that positive identification of a polydiacetylene can be made with reasonable certainty by detecting the triple bond vibration alone near 2100 cmxe2x88x921. They state also that positive identification can be made virtually certain by adding an additional criterion, viz., that the intensity of the Raman scattered light be nearly the same as that from a reference polydiacetylene sample. In the two examples of practical identification of the polydiacetylenes provided in the Patent, however, one can only find indication that xe2x80x9c . . . The scattered Raman light . . . was detected by a photomultiplier at the wavelength of maximum emission for the particular Raman scattering being tested.xe2x80x9d And, further: xe2x80x9cIn all the test prints containing the polymer, peaks due to the polydiacetylene backbone were readily observed. The vehicle, paper and colored pigments gave some overall general scattering, but of sufficiently low intensity not to obscure the polydiacetylene peaks.xe2x80x9d In another example the authors disclose that: xe2x80x9cSignal strength was measured at the point of maximum response. Non-Raman background scattering including fluorescence was subtracted from the total signal, to give that solely due to Raman, thus providing a unique document authentication system.xe2x80x9d
In the U.S. Pat. No. 5,718,754 (Pigment compositions, 1998, Macpherson et al., Ciba Specialty Chemicals) the use of coding compounds containing an azo, azomethine or polycyclic chromophore as Raman-detectable additives to pigments for security printing inks has been disclosed. These coding compounds are adsorbed on a printing pigment surface, or are used as a physical mixture with the pigment, in an amount sufficient to be detected by Raman or Resonance Raman Scattering (RRS) spectroscopy. Excitation with Argon Ion Lasers at wavelength of 514.5 nm or with different wavelengths in the red region obtainable with the aid of dye laser sources has been used for detection. As a pigment the authors suggest to employ any pigment commonly used in printing inks, such as arylamide, diarylide, azo metal salt, or phthalocyanine pigment. The coding compound adsorbed on the surface of the pigment is preferably a compound which is not normally used in printing inks, such as a copper phthalocyanine derivative. The coding compound which is physically mixed with the pigment is preferably an isoindolinone, diketopyrrolopyrrole, Schiff base metal complex, ferricyanide or a metal phtalocyanine derivatives. The authors note that the coding compound should preferably have an absorption frequency maximum at or near an absorption minimum of the pigment or even be outside the spectral range of the pigment. This separation gives the maximum sensitivity for detection by RRS. It is noted also that if the wavelength of the illuminating light is matched with the absorption maximum of the coding compound then the Raman spectrum recovered is significantly enhanced allowing much greater sensitivities to be obtained. It is further suggested that the matching of the wavelengths of the illuminating radiation and the absorption maximum of the coding compound can be achieved in two ways. Firstly, the laser wavelength may be selected to any devised wavelength and can therefore be used to detect the Raman spectrum of the coding compound in the presence of many different substances. Alternatively the coding compound can be selected so that it possesses an absorption maximum at or close to the available laser frequency. This latter option allows the lower cost system without loss of efficiency since tunable lasers are expensive. Although the authors note that the laser frequency can be visible, ultraviolet or infra-red when matched with suitably absorbing coding compounds, it is obvious that they rely on traditional RRS approach for detection. Not presenting factual exemplary spectra obtainable in accordance with their invention, Macpherson et al. provide only several Raman shift values for some of the peaks seen in the spectra they obtained. It is worth noting also that all the spectral information mentioned in the text of the patent has been acquired with the aid of expensive and non-portable Raman (Renishaw 2000 or an Anaspec modified Cary 81) spectrometers, rather fragile Argon Ion or expensive Dye lasers, and, deliberately, by using Resonance Raman Spectroscopy.
In another patent of Macpherson et al. (U.S. Pat. No. 5,853,464, Pigment compositions, 1998, Ciba Specialty Chemicals) use of Raman-detectable coding compounds that are adsorbed onto colloids or other nano scale metal particles is disclosed, and their use in printing inks for security applications such as banknotes and other security items is suggested. These coding compounds are detected by Surface Enhanced Resonance Raman Scattering (SERRS) spectroscopy to increase the intensity and detectability of the scattered light. According to the patent, the coding compounds may be a dye or a pigment and may be for example a phthalocyanine, a perinone, a quinacridone, an indanthrone, a flavanthrone, a pyranthrone, a perylene, a thioindigo, a dioxazine, an isoindoline, a diketopyrrolopyrrole, a basic dye complex, a metal complex, a monoazo, an azo metal salt, a disazo or a ferricyanide. Preferably coding compounds are used which have similar electronic absorption frequencies to the SERS plasmon resonance frequency. The authors recognize that Normal Raman scattering (NRS) is a very weak effect and gives with their materials low intensity signals, which can make detection difficult. So, in lines with traditional approach, they suggest to exploit tuning of the laser frequency to match an absorption maximum of the Raman-detectable compound, since scattering can be increased in efficiency by a factor of 103 to 104 due to resonance with the molecular electronic transitions. That is, they again rely on Resonance Raman scattering (RRS). To additionally increase the Raman signal intensity, it is suggested to exploit another known effect, the so called Surface Enhanced Raman Scattering (SERS). Surface enhancement arises from molecules which have been adsorbed onto a roughened surface, for example, roughened electrodes or aggregated colloids of nano-scale particles of SERS-active metals, such as silver, gold, copper, and lithium. Expectedly, surface enhancement can increase the intensity of the Raman scattering by up to 106. The size of the effect is dependent, however, on (i) the nature of the surface roughness, (ii) the distance of the Raman active molecules from the surface, (iii) the orientation of the molecules on the surface and (iv) the metal. Thus sensitivity varies widely depending on the exact nature of preparation of the surface or aggregated colloid and the method of addition of the Raman active molecules. Expecting that the scattering from a Raman active molecule on or near the surface of a SERS-active metal can be further enhanced when the laser light frequency is in resonance with an electronic transition of the Raman active molecule, the authors suggest to use the so called Surface Enhanced Resonance Raman Scattering (SERRS). They suggest that sensitivity of SERRS is much greater than the sum of SERS and RRS, and the identification of the Raman active molecule present is also extremely specific. Indeed, very low amounts of the Raman-detectable coding compound (between 0.1 and 100 ppm, preferably between 0.5 and 10 ppm) added of the SERS-active metal colloid are claimed in the patent. The authors further claim the following advantages of their SERRS system for use in security ink systems: (i) the sensitivity is much higher and there is a greater degree of selectivity with only vibrations from the coding compound being detected; (ii) in principle, photodecomposition could be minimized by SERRS since the energy transfer process between the coding compound and the surface reduces the lifetime of excited states and low powered lasers can be employed; (iii) SERRS can be used with both fluorescing and non-fluorescing compounds since fluorescence is quenched at the surface. It is further claimed that such an ink may be doped with extremely small quantities of the SERS-active metal aggregates of colloids known as the SERRS active sol. Then the ink can be printed by normal lithographic printing processes. It is claimed that the sensitivity is so great that unique, easily distinguished signals may be obtained from inks which contain so little of the specially prepared SERRS active sol that these inks are otherwise indistinguishable from clear varnish. The areas of surface required for examination can be as low as 1 xcexcm2, if a suitable microscope attachment is used. Nonetheless, although it is claimed that the coding compound which is adsorbed on the surface of the SERS active metal colloid may be selected from any compound which exhibits a characteristic Raman spectrum, those familiar with the art will appreciate that, up to date, only a very limited number of molecules giving useful SERRS spectra has been reported. The authors note that SERRS is preferred over SERS, where coding compounds are used which have a suitable electronic transition. But this condition in fact even further limits the choice of coding molecules. Another limitation is connected with the fact that, as the authors note, preferably the laser frequency is chosen to be close to that of the electronic transition and/or the frequency of the SERS plasmon resonance of the SERS-active metal colloid. It means the necessity to use expensive tunable lasers as excitation sources for the SERRS. Provided by the authors teaching that the coding compound should preferably be such that it gives a strong interaction with the SERS active metal colloid surface in fact additionally narrows the choice of molecules suitable for the role. It should be noted also that the known procedures of SERRS active sol preparation are rather tedious and plagued by poor reproducibility. Again, not presenting factual spectra permitting to evaluate, e.g., signal to noise ratio, resolution, and/or reproducibility of the obtainable spectra, the authors cite frequencies for several spectral bands without any assignments of the latter. Also, quoted spectra were acquired with the aid of expensive equipment. All this makes problematic a mass use of this system as a machine-readable security featurexe2x80x94especially in view of highly desirable adoption of Raman detection for computerized spectral search and control by unskilled operator xe2x80x9cin the fieldxe2x80x9d.
Concerning the Resonance Raman Spectroscopy approach deliberately used by the inventors in the above cited U.S. Pat. Nos. 5,324,567, 5,718,754 and 5,853,464, the disclosures of each of which are totally incorporated herein by reference, it is necessary to note the following. Indeed, until now in scientific circles as well as in patent literature the RRS, permitting to obtain a several orders of magnitude (ca. 104-106 times) enhancement of the Raman signal, sometimes is considered to be an advantageous technique compared to normal Raman scattering. However, this traditional approach, being based on use of laser excitation wavelengths falling in clear resonance with electronic or excitonic transitions characteristic for a compound to be detected, has serious drawbacks and limitations. For instance (and this is of prime importance in terms of the aims and spirit of our invention which will be disclosed later), the RRS (or SERRS) can hardly be rated as truly quantitative spectroscopic tools. The main reasons for this are: (i) the resonance Raman spectra are often contaminated or distorted by considerable thermal or fluorescent backgrounds caused by absorption of the laser frequency, the latter additionally being the cause for severe sample heating and/or degradation; (ii) intensity of the RR scattered light is not governed by the fundamental 4 law and is hardly predictable; (iii) shapes and intensities of the bands constituting the resonantly enhanced Raman spectra vary with the excitation wavelength used; (iv) when short-wave visible excitation is used, the generated Raman photons often have frequencies coinciding with spectral region of strong optical opacity of the sample, and hence reabsorption of the Raman photons by the sample unavoidably, and in an unpredictable manner, distorts the true intensity of the Raman bands. These and other less obvious shortcomings of the RRS approach will be discussed and illustrated in more details later (cf. DETAILED DESCRIPTION OF THE INVENTION section below), during discussion of essentials of the preferred by us xe2x80x9cnormalxe2x80x9d Raman approach.
There is also a patent in the prior art disclosing employment of non-resonance Raman spectroscopy for security purposes xe2x80x9cin the fieldxe2x80x9d (U.S. Pat. No. 5,818,047, Detector for explosive substances, 1998, Chaney et al., Renishaw P L C). According to the patent, Raman-active ingredients of some plastic explosives can be detected even in microquantities by virtue of analyzing traces of explosives in samples such as fingerprints left on a paper card. In this case the Raman-active ingredients of a plastic explosive Semtex (such as cyclotrimethylene-trinitramine, or RDX, and pentaerythrito-tetranitrate, or PETN) were detected by using an expensive research-grade Renishaw X20 Raman microscope (objective NA=0.45) and excitation with 25 mW HeNe laser emitting at 632.8 nm. The authors indicate that amount of power reaching the sample was about 5 mW, which corresponds to an energy density of 2xc3x97109 W/m2. Acquisition times of 5 s were sufficient for obtaining high quality spectra of RDX and PETN. The authors note, however, that both RDX and PETN appear to have large Raman scattering cross sections, making the Raman spectra easy to acquire. They have also tested a 514.5 nm line Ar+ laser with the same single crystalline samples and obtained essentially identical results. Moreover, the spectra were similar in appearance to those acquired using the FT Raman technique with 1064 nm Nd:YAG laser excitation. The tests of Chaney et al. have shown that their Raman apparatus and non-resonant approach can be successful in detecting extremely small particles of Semtex, e.g. about 1 xcexcm3, weighing about 1 picogram, in a few seconds. Their tests simulated real life conditions in which they made xe2x80x9cimpurexe2x80x9d samples with fingerprints which were cross contaminated by both Semtex and other greasy substances. The authors note that, in order to improve reliability of the detection, it may (among other possibilities) also be desirable to ensure that the boarding cards for taking fingerprints are made of a non-fluorescent (under 632.8 mn excitation) card material. An alternative to using non-fluorescent card they see in using a laser which produces light in the far red or in the infra-red region of the spectrum.
Of importance for comprehending aims, scope and advantages of the present invention is the fact that (as the patent of Chaney et al. Also surmises) there is no necessity always to provide Resonance Raman conditions to obtain good and useful Raman spectra of an analyte. Rather, Raman spectra of very high quality can be easily obtained also by using non-resonant or far from resonance excitations. In this case, however, the nature of a sample (i.e., whether this is an efficient Raman scatterer or not) is primarily responsible for whether intense or poor Raman spectrum will be produced. Hence, taking into account all the shortcomings of Resonance Raman spectroscopy noted above, it would be highly desirable to exploit an alternative approach based on normal Raman effect. The latter, however, although in principle being capable of providing more reliable, quantitative, practicable, simple and cost-effective means for detection of security features, automatically necessitates employment of intrinsically efficient Raman scatterers. Therefore, in accordance with the aims and scope of the present invention, there arises a substantial need for materials, coding compounds or taggants that would be intrinsically highly Raman-active, i.e. would have large Raman scattering cross sections and would be capable of producing intense Raman spectra under effect of characteristically non-resonant or far from resonance laser excitations.
For the sake of better understanding aims, scope, novelty and advantages of the present invention it seems necessary to analyze in more details teachings of the U.S. Pat. No. 5,935,755 (Method for document marking and recognition, 1999, Kazmaier et al., Xerox Corporation) the disclosure of which is totally incorporated herein by reference. This document discloses numerous marking materials comprising the so called Raman-detectable compounds suitable for employment in printing inks and xerography toners for security applications such as authentication of documents. Particularly preferred Raman-detectable components are considered to be those that exhibit a distinct Raman spectrum at a wavelength where most paper and dye or pigment colorants are transparent. More particularly, it is stated that, when exposed to a Nd:YAG laser at 1064 nm, many squaraine compounds emit a strong, unique signal in the Raman spectrum at about 1600 cmxe2x88x921 off the excitation laser line. Squaraine compounds preferred in the patent are of the general formula 
wherein R and R1 each, independently of the other, can be 
wherein A1 and A2 can each, independently of the other, be hydrogen atoms, alkyl groups, preferably with from 1 to about 18 carbon atoms, substituted alkyl groups, preferably with from 1 to about 18 carbon atoms, aryl groups, preferably with from about 6 to about 20 carbon atoms, substituted aryl groups, preferably with from about 6 to about 20 carbon atoms, arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, substituted arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, heterocyclic rings, preferably of from about 5 to about 7 members wherein the hetero atom is nitrogen, oxygen, sulfur, phosphorus, boron, or the like, such as pyrrolidine, pyridine, piperidine, piperazine, quinoline, isoquinoline, quinuclidine, pyrazole, triazole, tetrazole, triazine, imidazole, pyrimidine, pyradizine, pyrazine, oxazole, isoxazole, morpholine, and the like, halogen atoms, cyano groups, mercapto groups, hydroxy groups, amine groups, imine groups, ammonium groups, pyridine groups, pyridinium groups, ether groups, aldehyde groups, ketone groups, ester groups, amide groups, carboxylic acid groups, carbonyl groups, thiocarbonyl groups, sulfate groups, sulfonate groups, sulfide groups, sulfoxide groups, phosphine groups, phosphonium groups, phosphate groups, cyano groups, nitrile groups, mercapto groups, nitroso groups, nitro groups, sulfone groups, acyl groups, acid anhydride groups, azide groups, and the like, and wherein A1 and A2 can be joined together to form a ring containing the nitrogen atom to which A1 and A2 are attached, such as a pyrrole ring, an indole ring, an indolizine ring, a pyrrolidine ring, a pyridine ring, a piperidine ring, a piperazine ring, a quinoline ring, an isoquinoline ring, a quinuclidine ring, an indazole ring, a pyrazole ring, a triazole ring, a tetrazole ring, a triazine ring, a urazole ring, an imidazole ring, a pyrimidine ring, a pyradizine ring, a pyrazine ring, an oxazole ring, an isoxazole ring, a morpholine ring, or the like, and wherein B1, B2, B3, and B4 can each, independently of the other, be hydrogen atoms, alkyl groups, preferably with from 1 to about 18 carbon atoms, substituted alkyl groups, preferably with from 1 to about 18 carbon atoms, aryl groups, preferably with from about 6 to about 20 carbon atoms, substituted aryl groups, preferably with from about 6 to about 20 carbon atoms, arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, substituted arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, heterocyclic rings, preferably of from about 5 to about 7 members wherein the hetero atom is nitrogen, oxygen, sulfur, phosphorus, boron, or the like, such as pyrrolidine, pyridine, piperidine, piperazine, quinoline, isoquinoline, quinuclidine, pyrazole, triazole, tetrazole, triazine, imidazole, pyrimidine, pyradizine, pyrazine, oxazole, isoxazole, morpholine, and the like, halogen atoms, cyano groups, mercapto groups, hydroxy groups, amine groups, imine groups, ammonium groups, pyridine groups, pyridinium groups, ether groups, aldehyde groups, ketone groups, ester groups, amide groups, carboxylic acid groups, carbonyl groups, thiocarbonyl groups, sulfate groups, sulfonate groups, sulfide groups, sulfoxide groups, phosphine groups, phosphonium groups, phosphate groups, cyano groups, nitrile groups, mercapto groups, nitroso groups, nitro groups, sulfone groups, acyl groups, acid anhydride groups, azide groups, and the like, wherein the B groups can be joined together to form one or more rings, 
wherein G is selected from the group consisting of hydrogen atoms, alkyl groups, preferably with from 1 to about 18 carbon atoms, substituted alkyl groups, preferably with from 1 to about 18 carbon atoms, aryl groups, preferably with from about 6 to about 20 carbon atoms, substituted aryl groups, preferably with from about 6 to about 20 carbon atoms, arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, substituted arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, heterocyclic rings, preferably of from about 5 to about 7 members wherein the hetero atom is nitrogen, oxygen, sulfur, phosphorus, boron, or the like, such as pyrrolidine, pyridine, piperidine, piperazine, quinoline, isoquinoline, quinuclidine, pyrazole, triazole, tetrazole, triazine, imidazole, pyrimidine, pyradizine, pyrazine, oxazole, isoxazole, morpholine, and the like, halogen atoms, cyano groups, mercapto groups, hydroxy groups, amine groups, imine groups, ammonium groups, pyridine groups, pyridinium groups, ether groups, aldehyde groups, ketone groups, ester groups, amide groups, carboxylic acid groups, carbonyl groups, thiocarbonyl groups, sulfate groups, sulfonate groups, sulfide groups, sulfoxide groups, phosphine groups, phosphonium groups, phosphate groups, cyano groups, nitrile groups, mercapto groups, nitroso groups, nitro groups, sulfone groups, acyl groups, acid anhydride groups, azide groups, and the like, and D1, D2, D3, D4, D5, D6, and D7 each, independently of the others, can be hydrogen atoms, alkyl groups, preferably with from 1 to about 18 carbon atoms, substituted alkyl groups, preferably with from 1 to about 18 carbon atoms, aryl groups, preferably with from about 6 to about 20 carbon atoms, substituted aryl groups, preferably with from about 6 to about 20 carbon atoms, arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, substituted arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, heterocyclic rings, preferably of from about 5 to about 7 members wherein the hetero atom is nitrogen, oxygen, sulfur, phosphorus, boron, or the like, such as pyrrolidine, pyridine, piperidine, piperazine, quinoline, isoquinoline, quinuclidine, pyrazole, triazole, tetrazole, triazine, imidazole, pyrimidine, pyradizine, pyrazine, oxazole, isoxazole, morpholine, and the like, halogen atoms, cyano groups, mercapto groups, hydroxy groups, amine groups, imine groups, ammonium groups, pyridine groups, pyridinium groups, ether groups, aldehyde groups, ketone groups, ester groups, amide groups, carboxylic acid groups, carbonyl groups, thiocarbonyl groups, sulfate groups, sulfonate groups, sulfide groups, sulfoxide groups, phosphine groups, phosphonium groups, phosphate groups, cyano groups, nitrile groups, mercapto groups, nitroso groups, nitro groups, sulfone groups, acyl groups, acid anhydride groups, azide groups, and the like, and wherein two or more of the G and D groups can be joined together to form one or more rings, 
wherein E1 and E2 each, independently of the other, can be hydrogen atoms, alkyl groups, preferably with from 1 to about 18 carbon atoms, substituted alkyl groups, preferably with from 1 to about 18 carbon atoms, aryl groups, preferably with from about 6 to about 20 carbon atoms, substituted aryl groups, preferably with from about 6 to about 20 carbon atoms, arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, substituted arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, heterocyclic rings, preferably of from about 5 to about 7 members wherein the hetero atom is nitrogen, oxygen, sulfur, phosphorus, boron, or the like, such as pyrrolidine, pyridine, piperidine, piperazine, quinoline, isoquinoline, quinuclidine, pyrazole, triazole, tetrazole, triazine, imidazole, pyrimidine, pyradizine, pyrazine, oxazole, isoxazole, morpholine, and the like, halogen atoms, cyano groups, mercapto groups, hydroxy groups, amine groups, imine groups, ammonium groups, pyridine groups, pyridinium groups, ether groups, aldehyde groups, ketone groups, ester groups, amide groups, carboxylic acid groups, carbonyl groups, thiocarbonyl groups, sulfate groups, sulfonate groups, sulfide groups, sulfoxide groups, phosphine groups, phosphonium groups, phosphate groups, cyano groups, nitrile groups, mercapto groups, nitroso groups, nitro groups, sulfone groups, acyl groups, acid anhydride groups, azide groups, and the like, and F1, F2, F3, F4, and F5 each, independently of the others, can be hydrogen atoms, alkyl groups, preferably with from 1 to about 18 carbon atoms, substituted alkyl groups, preferably with from 1 to about 18 carbon atoms, aryl groups, preferably with from about 6 to about 20 carbon atoms, substituted aryl groups, preferably with from about 6 to about 20 carbon atoms, arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, substituted arylalkyl groups, preferably with from about 6 to about 38 carbon atoms, heterocyclic rings, preferably of from about 5 to about 7 members wherein the hetero atom is nitrogen, oxygen, sulfur, phosphorus, boron, or the like, such as pyrrolidine, pyridine, piperidine, piperazine, quinoline, isoquinoline, quinuclidine, pyrazole, triazole, tetrazole, triazine, imidazole, pyrimidine, pyradizine, pyrazine, oxazole, isoxazole, morpholine, and the like, halogen atoms, cyano groups, mercapto groups, hydroxy groups, amine groups, imine groups, ammonium groups, pyridine groups, pyridinium groups, ether groups, aldehyde groups, ketone groups, ester groups, amide groups, carboxylic acid groups, carbonyl groups, thiocarbonyl groups, sulfate groups, sulfonate groups, sulfide groups, sulfoxide groups, sulfate groups, sulfonate groups, phosphine groups, phosphonium groups, phosphate groups, cyano groups, nitrile groups, mercapto groups, nitroso groups, nitro groups, sulfone groups, acyl groups, acid anhydride groups, azide groups, and the like, wherein two or more of the E and F groups can be joined together to form one or more rings, and the like.
Also preferred as Raman-detectable components in frames of the U.S. Pat. No. 5,935,755 are named phtalocyanines of the general formula 
wherein M represents a metal atom, such as copper or the like, a functionalized metal, that is, any metal bearing additional substituents or ligands beyond the phthalocyanine moiety, such as oxovanadium, hydroxygallium, or the like, wherein bonding can be either predominantly ionic or covalent in nature, or two hydrogen atoms (metal-free phthalocyanine). Also preferred as Raman-detectable components are considered to be metal dithiolates, of the general formula 
wherein M represents a transition metal atom, with nickel being preferred, and wherein R1, R2, R3, and R4 each, independently of the others, can be an alkyl group, preferably with from about 1 to about 18 carbon atoms, a substituted alkyl group, preferably with from about 1 to about 18 carbon atoms, an aromatic group, preferably with from about 6 to about 20 carbon atoms, a substituted aromatic group, preferably with from about 6 to about 20 carbon atoms, an arylalkyl group, preferably with from about 6 to about 38 carbon atoms, a substituted arylalkyl group, preferably with from about 6 to about 38 carbon atoms, or a heterocyclic ring, preferably with from about 5 to about 7 members, with the hetero atom being nitrogen, oxygen, sulfur, phosphorus, boron, or the like, with examples of suitable substituents including but not limited to mercapto groups, hydroxy groups, amine groups, imine groups, ammonium groups, pyridine groups, pyridinium groups, ether groups, aldehyde groups, ketone groups, ester groups, amide groups, carboxylic acid groups, carbonyl groups, thiocarbonyl groups, sulfate groups, sulfonate groups, sulfide groups, sulfoxide groups, sulfate groups, sulfonate groups, phosphine groups, phosphonium groups, phosphate groups, cyano groups, nitrile groups, mercapto groups, nitroso groups, nitro groups, sulfone groups, acyl groups, acid anhydride groups, azide groups, and the like, with examples of preferred groups including (but not limited to) phenyl, indole, furan, pyrrole, and the like, and wherein two or more of the R groups can be joined to form a ring.
Thus, according to the U.S. Pat. No. 5,935,755, the numerous disclosed Raman-detectable compounds, when irradiated with monochromatic radiation in the near infrared region of the spectrum, exhibit a detectable Raman spectrum which can be measured using standard FT-Raman instruments. Preferably, the Raman-detectable component is one that gives a Raman spectrum when irradiated with light in the near-infrared region, more particularly in the wavelength range of from about 680 to about 1400 nm. The authors recognize that Raman-detectable components which emit a Raman signal at excitation wavelengths lower than about 680 nm are also suitable, but may be less preferred because papers, toner pigments, ink colorants, and other similar materials which may also be present on or in the recording sheet may also absorb energy at these lower wavelengths, which might mask the Raman signal of the Raman-detectable component. Raman-detectable components which emit a Raman signal at wavelengths higher than about 1400 nanometers are also suitable, but may be difficult to obtain.
In a typical example, a squaraine compound [bis(4-dimethylamino-2-phenyl)-squaraine] (9) in powdered form was applied to a sheet of a plain paper as a small dot by pressing the material into the paper with a spatula and removing loose material by gently blowing the area with a stream of nitrogen. The paper thus treated was placed into the sample compartment of a Perkin-Elmer 1760 NIR-FT Raman spectrometer. To obtain the spectrum, the paper was aligned so that the Nd:YAG laser impinged on the treated spot. The Raman signal obtained from the treated spot was compared to the signal obtained from an untreated spot on the same paper, and a signal characteristic of the squaraine compound was recorded. In other examples the same approach has been used for marking a polyester film, a glass surface and an aluminium plate. However, despite the numerous compounds are quoted in the disclosure, in fact the authors give examples of preparing toners and inks for ink-jet printers comprising only the squaraine compounds of the few specific formulae below: 
In additional examples, dry toner compositions suitable for developing electrostatic images, inks suitable for ink-jet printers, a UV-curable liquid developer suitable for polarizable liquid development processes and an electrophoretic developer were prepared comprising bis(4-dimethylamino-2-hydroxy-6-methylphenyl)squaraine or bis(4-dimethylamino-3-methoxy-phenyl)-squaraine (7) compounds as the Raman-detectable component in different proportions to other ingredients of the toner, ink or developer compositions. In all given examples, however, the developed images were rendered by the authors to be Raman-detectable without providing either factual NIR-FT Raman spectra or any indication for the Raman bands frequencies, not saying of the laser powers used or of intensity characteristics for different Raman bands if any. Instead, in all these instances the authors state that: xe2x80x9cThe Raman signal obtained from the treated spot is compared to the signal obtained from an untreated spot on the same substrate, and it is believed that a strong signal characteristic of the squaraine compound will be recorded.xe2x80x9d
Not questioning here the disclosed in the U.S. Pat. No. 5,935,755 facts of observing Raman spectra for several squaraine compounds with the aid of a research-grade NIR-FT Raman instrument, it is necessary to note the following.
(a) Yet 10 years ago, Raman spectroscopy has been used exclusively in research labs, almost never in analytical ones and positively never xe2x80x9cin the fieldxe2x80x9d. The reasons are color and fluorescence problems that frequently arise when conventional shorter wavelength (visible) excitation is used. Colored samples absorb the source radiation and prevent one from seeing the Raman scatter, while even very weak fluorescence is much stronger than Raman scatter and effectively drowns it out. According to estimates of those familiar with the art, less than 20% of the vast range of samples studied up to date with conventional (visible) excitations have given observable Raman spectra; industrial analytical labs that used blue, green and red lasers reported similar success rates with consequently high costs per spectrum. At the beginning of 90 several main manufacturers of Fourier transform (FT) infrared spectrometers announced marketing of their near infrared (NIR) FT Raman spectrometers. The idea in NIR-FT Raman spectroscopy is to excite the spectrum with a near infrared laser, pass the scattered light through filters to remove the reflected and Rayleigh scattered excitation radiation, and then to process the Raman scatter on a Fourier transform infrared (FTIR) instrument. The pioneers used a range of research grade FTIRs and have developed filter and detector technologies allowing excellent spectra to be recorded in the near infrared. Being infrared excited, the spectra were virtually clear of fluorescence, while color (due to absorption of laser wavelength) was far less of a problem in near infrared than with conventional techniques. Thus, the following major advances have been achieved: (i) almost all tried samples give some sort of Raman spectrum; (ii) fluorescence and color present minimal problems; (iii) the quality of spectra previously difficult to study is frequently exceptional. Turning now to the preferred embodiments of the U.S. Pat. No. 5,935,755, it should be emphasized that all experiments described therein have been performed with the aid of research-grade (and hence of high-quality, but expensive) Perkin-Elmer 1760 or BOMEM Corporation (Maarssen, Netherlands) NIR-FT Raman spectrometers. However, as practitioners in the art will probably appreciate (and as it has been indicated above), almost all tried samples give some sort of Raman spectrum with the aid of such high-end instruments. Those familiar with the art will recognize also that, in order for provide a reliable Raman spectroscopic detection (authentication) of molecular security labels or markings, the tagging compounds are highly desirable which not only would be Raman-detectable, but would be strong Raman scatterers. Being devoid a possibility to evaluate the Raman-scattering ability (intensity of the corresponding Raman spectra) of the compounds disclosed in the U.S. Pat. No. 5,935,755 from the text, one is free to believe that the squaraine compounds of the patent may be only moderately Raman-active by their nature. This suggestion was supported by the results of comparative experiments constituting an integral part of the present invention (cf. DETAILED DESCRIPTION OF THE INVENTION section below) in which superior Raman-active compounds are disclosed and counterposed to those ones suggested in frames of the U.S. Pat. No. 5,935,755.
(b) Until now NIR-FT Raman spectrometers have not find considerable employment in so called xe2x80x9cfieldxe2x80x9d measurements of samples from real life. The main reasons are (i) comparative fragility of the interferometer and hence its disaligment problems, leading to worsening the signal to noise ratio (SNR) of collected spectra; and (ii) rather high cost of the desktop instruments based mainly on research-grade FTIRs. Hence, being acquired with the aid of expensive and fragile laboratory instruments, teachings of the U.S. Pat. No. 5,935,755 are hardly directly transferable for measurements xe2x80x9cin the fieldxe2x80x9d which would be made with the aid of less expensive dispersive Raman spectrometers.
(c) Those familiar with the art will recognize that, in order to perform spectroscopic detection (authentication) of security labels or markings xe2x80x9cin the fieldxe2x80x9d and in mass application, a Raman spectrometer would need to be rugged, sensitive, stable and inexpensive. One would need a reasonable optical resolution and a wide spectral range. Remote sampling through fiber optics would be a great advantage, allowing access to parts, articles and to environments which could not be reached by a conventional desktop or laboratory instrument. The ideal instrument would need only electricity to operate, with no requirements for cooling water, cryogens or other services. From a spectropists point of view, it would be desirable to work with strong fundamental vibrational bands rather than overtones and combinations. In principle, NIR-Raman spectroscopy can answer all of the needs on the above list, but until now the technique has been restricted to the laboratory because instrumentation was not sufficiently rugged. The renaissance in Raman spectroscopy, begun by the development of FT-Raman instruments, is now being further boosted by significant advances in dispersive Raman technology. Recent and continuing improvements in solid state lasers, filters and detectors have made dramatic improvements in both the ruggedness and sensitivity of dispersive Raman spectrometers. As a result, truly portable, rugged, sufficiently sensitive, and rather economic dispersive Raman spectrometers are feasible now which can find mass employment in the field for security authentication purposes. Nonetheless, those skilled in the art would appreciate that, for exploiting machine-readable technologies for security authentication purposes, it would be advantageous (in the same fashion as humans for long devised, optimized and used more bright, more contrast and hence more clearly visible labels, marks and signs for different self-needs and convenience) to deliberately devise, produce and use machine-readable tags, marks and labels which, while sometimes being rather dull or even completely invisible for unaided human eye, would be highly bright, highly contrast, more easy and xe2x80x9cconvenientxe2x80x9d for the machine to detect and to identify. We recognized (and this notion has been laid down in the grounds of the present invention) that, on this way, it is imperative to try to provide as easy tasks for a detecting machine as possible. It is supposed also that these efforts will be compensated later by thus achievable higher reliability and convenience of the authentication process as well as by costs savings for the instrumentation, since the easier the task the more simple the detecting machine can be. As it will be illustrated later (cf. DETAILED DESCRIPTION OF THE INVENTION section below) by way of comparative experiments, besides those NIR-FT Raman-detectable compounds which has been disclosed in the U.S. Pat. No. 5,935,755, and which should be rendered in fact as moderately Raman-active ones, there exists a plenty of superior Raman-active compounds which has been deliberately devised in lines with the just mentioned philosophy and which are Raman-active in such extent that can be easily detected and authenticated xe2x80x9cin the fieldxe2x80x9d even with the aid of simplest dispersive Raman spectrometers using a low-power NIR laser as the excitation source.
(d) Practitioners in the art will probably recognize that the Raman-detectable compounds of the U.S. Pat. No. 5,935,755 have another, less obvious but essential in the long run, drawback. More close analysis of chemical formulas of the compounds preferred in the patent shows that aromatic or heteroaromatic cycles along with the squaraine cycle and a short chain of conjugated double bonds are the main structural blocks for the most of the compounds suggested for the role of a Raman-detectable marking. These will reveal their most prominent characteristic Raman bands in the region of 1650-1300 cmxe2x88x921. At the same time, despite the apparent multiformity of the numerous disclosed in the patent substituents and groupings bearing different chemical names, most of them will also exhibit their most prominent characteristic Raman bands in the same Raman shifts region. In other words, it is expected that, frequently, the spectral region of 1650-1300 cmxe2x88x921 will be overcrowded with spectral features. The latter circumstance will tend to result in an ensemble of poorly resolved and/or superimposed bands with broad shoulders instead of a clear sequence of well resolved spectral features, thus worsening reliability of the authentication process or calling for necessity to work at higher resolutions or using longer scanning times or both. This drawback could be especially troublesome during attempts to employ mixtures of different compounds from the lot disclosed in the U.S. Pat. No. 5,935,755xe2x80x94e.g., in order to increase the number of available authenticating markings and codes.
(e) Another potential drawback of the Raman-detectable compounds disclosed in the U.S. Pat. No. 5,935,755 is connected with the fact that these compounds are rather xe2x80x9cphoto-activexe2x80x9d. Those familiar with the art will recognize that squaraines, also called Squarylium dyes, for some time are being marketed as near infrared photoreceptors for laser printers. For instance, according to an article entitled xe2x80x9cNear-Infrared Absorbing Dyes,xe2x80x9d Chemical Reviews, 1992, No. 6, 1197-1226 (July 1992) by Fabian et al., a symmetrical Squarylium exhibits photoconductivity under irradiation with light at a wavelength of 830 nm. Although this article is silent on the subject of fluorescence for Squarylium dyes, their fluorescence characteristics are mentioned and exploited in U.S. Pat. No. 5,928,954 (Tagging hydrocarbons for subsequent identification, Jul. 27, 1999, Rutledge, et al., BP Amoco Corporation), the disclosure of which is totally incorporated herein by reference. In particular, this patent states that Squarylium dyes (or squaraines) constitute a preferred group of fluorescent dyes (along with naphthalocyanine, phthalocyanine, cyanine, methine and croconium dyes) due to the ability of these effectively absorb and fluoresce at wavelength in the range of about 600 to 2500 mn. Hence, although there is no mentioning about fluorescence of squaraines under Nd:YAG laser in the U.S. Pat. No. 5,935,755, there are strong chances to induce fluorescence for squaraines by using other NIR laser frequencies. For instance, probability to produce fluorescence instead of Raman signal with squaraines will be especially high during attempts to employ many popular diode lasers (such as those lasing at 780 nm, 808 nm, 830 nm and, perhaps, 960 nm). Practitioners will recognize that the latter possibility is able to further limit applicability of the squaraine derivatives as Raman-detectable security markings. Alternatively, the same circumstance may lead to necessity of using more expensive InGaAs matrix or array detectors instead of more affordable silicon CCD""s in corresponding dispersive Raman instrumentation suitable for identification of squaraines security markings.
(f) Finally, those familiar with the art will probably recognize that the Raman-detectable compounds of the U.S. Pat. No. 5,935,755 has another principal drawback. As is well known, in order for a molecular vibration to be Raman active, the vibration must be accompanied by a change in the polarizability of the molecule. The polarizability can be looked on as the deformability of the electron cloud of the molecule by the electric field (or the ease with which electrons can be induced to respond to a potential gradient). Roughly, probability of polarizability changes increases with symmetry and electron density between the oscillating masses. Raman scattering intensity depends upon the degree of modulation of the polarizability of the scattering species during a vibrational cycle, i.e., Raman frequencies arise from changes in the electronic polarizability associated with nuclear vibrational displacements. Thus symmetric vibrational modes of symmetric species and groups which contain polarizable bonds such as, e.g., I2, H2, xe2x80x94Sxe2x80x94Sxe2x80x94,  greater than Cxe2x95x90C less than , xe2x80x94CN tend to scatter strongly (have relatively large Raman scattering cross-section). At the same time, since polar species have larger dipole moments than more symmetric molecular fragments, strong IR-absorption spectral features arise from vibrations of the asymmetric groups. And vice versa. Thus, as a rule of thumb, those normal vibrations which are highly active in the infrared absorption spectroscopy are usually less Raman-active, and vice versa. Turning again to the compounds disclosed in the U.S. Pat. No. 5,935,755, it may be seen that most of them, e.g. molecules (1)-(4) and (7)-(9) are asymmetrical. Also, an aromatic or phenyl ring is the most symmetrical element in these molecular structures, while ethene or aromatic double Cxe2x95x90C bond is the most polarizable individual chromophore. Although the disclosed molecules posses some degree of conjugation (comprise a system of conjugated unsaturated chromophores within the molecule), and the latter is known to increase the Raman cross-section of a compound, the conjugation length, at best, is limited to 3-5 double bonds in these cases. And the so called effective conjugation length, in fact responsible for the Raman signal enhancement, should be much shorter in these systems because of geometrical factors. Further, according to the list of the side substituents to the main structures (1)-(4), the most of the groupings disclosed therein are either highly asymmetrical or polar by nature. More specifically, only few groups such as, e.g., piperazine (10), pyrazine (11), cyano group (12), imine group (13), nitro group (14), and sulfone group (15) 
can expectedly exhibit more or less prominent Raman signals, while the rest, being active in infrared absorption spectroscopy, should be relatively Raman-inactive due to their asymmetry or polarity or both. Certainly, some of these molecules have some necessary attributes capable to impart them moderate Raman activity, but they hardly can have very high Raman scattering cross-section. Rather, those familiar with the art will probably appreciate that, in terms of their Raman activity, these disclosed compounds should be very close to ordinary aromatic, heteroaromatic or condensed aromatic molecular structures. Being devoid a possibility to assess the Raman-activity of the disclosed compounds due to the absence of factual spectra in the patent text, one is free to believe that the squaraine compounds of the U.S. Pat. No. 5,935,755 are only moderately Raman-active. This suggestion has been supported by the results of comparative experiments (cf. DETAILED DESCRIPTION OF THE INVENTION section below) in which superior Raman-active compounds of the present invention are disclosed and counterposed to those ones suggested in frames of the U.S. Pat. No. 5,935,755.
Thus, the analysis undertaken above suggests that, in the increasingly complex world of machine-assisted handling of paper and other values protected with special anti-forgery machine-readable marks and tags there is still a need for improved marking/recognition systems based on Raman spectroscopy.
It is apparent that a need exists for a Raman-spectroscopy-based methodology which would provide a reliable detection and authentication of different taggants without extensive sample preparation by means of truly quantitative, normal Raman scattering spectra excited far from electronic or excitonic resonances.
It is apparent also that it would be advantageous to provide a small, hand-held or portable Raman spectrometer having sufficient resolution and accuracy for use in a wide variety of applications relevant to verification or authentication of different security marks.
It is obvious further that a need remains for a low-cost miniaturized spectrometric sensor/transducer with a spectral resolution comparable to that of conventional Raman spectrometers, and capable of quantitative determining the Raman spectral signatures of a wide variety of substances xe2x80x9cin the fieldxe2x80x9d.
It is apparent also that, although known Raman-detectable compounds and corresponding processes exist in the art which are suitable for their intended purposes, a need still remains for more reliable, simplified, expressive and economic methods of detecting and authenticating the security marks and tags by means of Raman spectroscopy.
It is apparent also that a need remains for marking materials capable, under non-resonant or far from resonance laser excitation, of generating intense and well resolved Raman spectra which could be easily detected, identified and/or authenticated by simple, rugged and affordable instruments directly xe2x80x9cin the fieldxe2x80x9d.
It is apparent further that there is a substantial need for materials, coding compounds or taggants that are intrinsically highly Raman-active, i.e. have large Raman scattering cross sections, and are capable of producing intense Raman spectra under effect of characteristically non-resonant or far from resonance laser excitations.
Further, there is a need for producing a multiplicity of widely diversified Raman-active marking materials that would permit to generate an unlimited number of truly unique authenticating codes for fail-safe authentication of the items protected therewith.
Even further, it is apparent that there exist a significant need in the art for both tagging materials and detection methods that permit sensitive optical and/or spectroscopic detection of specific taggant signal(s) with essentially total rejection of nonspecific background noise and which are nondestructive for different kinds of substrates carrying the labels.
Finally, it is apparent that there is a need for new and diversified methods for marking documents that would be very difficult to duplicate or forge.
It is the main object of the present invention to provide an improved methodology of prefatory authenticity protection and consecutive machine-assisted authentication of genuine documents and other values which need to be protected against forgery and/or counterfeiting and which may be represented by: (1) traditional substrates used for printing of documents, such as different types of paper, (2) different types of plastics used for producing financial instruments, such as bankcard-grade PVC and the like, (3) multitudinous other articles bearing security printing which can include banknotes, banknote thread, currency, traveler""s"" checks, bonds, certificates, stamps, lottery tickets, ownership documents, passports, identity cards, credit cards, charge cards, access cards, smart cards, brand authentication labels and tags, tamperproof labels and the like. Instrumentally, this methodology relies upon Raman spectrometry in the near infrared region of the electromagnetic spectrum.
In brief, the essence of the invention may be reduced to the principle: xe2x80x9cBefore trying to perform authentication of a genuine item among fakes by means of Raman spectroscopy, equip the genuine item with a molecular tag, marking or label incorporating special chemicals capable of giving intense Raman spectra under NIR laser excitation. Then the authentication of the tagged genuine values among fakes can be reliably performed by using even extremely simple and economical spectrometers.xe2x80x9d
It is another object of the present invention to provide highly reliable processes for machine-assisted authentication of values and documents.
It is yet another object of the present invention to provide simple, rugged and affordable spectrometric means for authenticating genuine items marked with such materials xe2x80x9cin the fieldxe2x80x9d.
It is still another object of the present invention to provide the processes for placing the efficient Raman-active materials of the invention on genuine items.
It is another object of the present invention to provide methods for covert marking of values and documents that would be very difficult to duplicate or forge.
It is another object of the present invention to provide multiplicity of highly diversified chemical compounds capable of emitting strong Raman spectra under effect of near infrared laser irradiation.
It is another object of the present invention to provide thermochromic marking materials capable of generating chromatic images that are visible and which can be identified and distinguished from other visible marking materials by means of both visual inspection and spectrometric techniques.
These and other objects of the present invention are achieved by providing a process which comprises (a) applying to a genuine item to be protected against duplicating, forgery or counterfeiting a marking material comprising an efficient Raman-active compound (or a mixture of several such compounds) which, when irradiated with monochromatic radiation from the near infrared region of electromagnetic spectrum, exhibits a detectable Raman spectrum, thereby forming a molecular code or security mark on the genuine item; (b) irradiating the security mark on the such protected genuine item with monochromatic radiation from the near infrared region of electromagnetic spectrum; (c) measuring the Raman spectrum of radiation scattered from the security mark when the mark is irradiated with monochromatic radiation belonging to the near infrared region of electromagnetic spectrum; and (d) processing the spectral data so obtained with the aid of corresponding dedicated software and presenting the results of the molecular code recognition to an operator in an unambiguous and convenient form.
In terms of materials science, the present invention is based on the discovery that several specific classes of chemical compounds comprising:
(i) essentially monomeric linear, quasi-linear or branched organic and organoelement compounds containing in terminal or internal positions within a molecule essentially symmetrical molecular moieties (or chromophores) having intrinsically high polarizabilities such as, e.g., HCxe2x89xa1Cxe2x80x94, HCxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94,  greater than Cxe2x95x90Nxe2x80x94, xe2x80x94Nxe2x95x90Nxe2x80x94,  greater than Cxe2x95x90C less than , xe2x80x94Cxe2x89xa1Cxe2x80x94 Cxe2x89xa1Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94Phxe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Phxe2x80x94Cxe2x89xa1Cxe2x80x94Phxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 Cxe2x89xa1Cxe2x80x94Pdxe2x80x94Cxe2x89xa1Cxe2x89xa1Cxe2x80x94,  greater than Cxe2x95x90Cxe2x80x94Cxe2x95x90C less than , etc.;
(ii) essentially polymeric linear or quasi-one-dimensional, ladder-like or comb-like, highly branched or dendritic macromolecular compounds containing long sequences of essentially symmetrical unsaturated chromophores in their backbones exhibiting intrinsically high polarizabilities and/or characterized by extensive delocalization of electron density along the conjugated backbone such as, for example, polyenes, polyenes, polysilylenes, polyenynes, poly(arylenevinylenes), poly(arylenevinyleneethynylenes), etc., along with new allotropic forms of carbon or quasi-zero-dimensional spheroidal large carbon molecules such as, for example, fullerness C60, C70, nanotubes and the likexe2x80x94all readily produce strong Raman spectra essentially devoid of autofluorescence or other interfering backgrounds.
(iii) lanthanide-ions-based inorganic up-converting phosphors, being used as such, as mixtures of one with another or as mixtures with organic Raman-active compounds, can constitute an unusual class of improved machine-readable taggants. Although the use of such compounds here is based on exploitation of Laser Induced Fluorescence (LIF) phenomenon rather than of normal Raman scattering, in frames of the invention the inorganic upconverting phosphors are tentatively qualified as efficient Raman-detectable taggants in view of the fact that they can be conveniently detected and identified with the aid of the very same equipment which normally is used for detecting the organic Raman-active compounds of the above classes (i) and (ii) in accordance with the invention.
As it has been discovered by the inventors in the course of extensive studies, materials of these three classes are amenable to be used as highly efficient Raman-active or Raman-detectable components of security codes, markings and tags that can be conveniently detected, identified and authenticated by their normal Raman and/or LIF/Raman spectra excited with the aid of near-infrared lasers.
In some embodiments, at least, the invention is based on the discovery that the above chemical compounds, being subjected to excitation by near-infrared lasers, reveal unusually high Raman scattering activity, making them outstanding or unique among many other classes of chemicals offered for producing Raman-detectable security marks in the prior art.
The effective Raman-active taggants and corresponding detection methods of the invention provide essentially total rejection of non-specific background emissions such as autofluorescence and/or thermal backgrounds. Of prime importance for practicing the invention is the fact that (contrary to teachings of the prior art that often rely on using visible lasers for inducing the RRS or SERRS phenomena) employment of NIR-laser excitations with compounds of the types disclosed in the present invention results in a number of essential advantages. Being based on employment of the pre-resonant, non-resonant or far-from-resonance laser excitations, the methods of the invention avoid the adverse effects of visible lasers upon the tagging materials. Also, due to preventing undesirable processes of molecular structure transformations or degradation of the taggants in the course of the detection procedure, grounds for true quantitative measurements are provided. Owing to this, for instance, detection of multiple Raman-active compounds of the invention in complex mixtures thereof becomes possible.
In contrast to Raman-detectable compounds of the types described in the previous art, those of the present invention are found to be equally useful as efficient and reliable Raman-detectable taggants when used incorporated in both either clearly visible or completely hidden from the eye markings, patterns and images. The detection of the hidden security markings becomes feasible in accordance with the invention due to the possibility to employ covering layers which, despite opaque in the visible range, can be fairly transparent to both NIR laser excitation wavelength and to the corresponding Raman photons.
Being based on the discovered ability of the disclosed chemicals to effectively generate Raman photons under NIR excitation, the present invention provides materials and detection methods which permit ultrasensitive detection and identification of the corresponding molecular tags, codes and markings and, thus, provides improved reliability of authentication of genuine items protected therewith.
In one embodiment, at least, the present invention is based on the discovery that containing lanthanide ions compounds exhibiting two-photon upconverting ability are of value as components of highly covert molecular labels and codes amenable to simple and reliable identification with the aid of the same instruments which are used for measuring normal Raman spectra of the other compounds of the invention, but with the aid of diversified light sources.
In several embodiments, inks, paints and other compositions of the invention incorporating the disclosed efficient Raman-active compounds can normally be used in addition to/beside security-printed areas in a variety of colors and can be applied by offset lithographic, screen or flexographic processes.
The Raman-active compositions of the invention may also be included in electro-photographic toners, matrix or daisy-wheel printer inks, and may be used in non-impact printing methods such as ink-jet printing or drafting and drawing with different writing tools such as pens, fountain-pens, flow-masters, permanent markers, plotter pens, roller pens, capillary pens, gel pens, etc.
The Raman-active compounds of the invention may also be included, not necessarily as inks, in paper including rag papers and plastic papers, banknote threads, plastic cards and other security documents or items which need to be authenticated, if necessary blended with a polymer and bonded other than in an ink. The Raman-active compounds of the invention may be deposited in a single area or a series of areas, if necessary or desired in a coded pattern. Moreover, the Raman-active compounds and compositions of the invention may be incorporated into other items which need to be authenticated, e.g. by incorporating it in a label such as a holographic label bearing printing in an ink containing a Raman-active compound, or in a hot-stamping foil construction.
The nature of the invention provides considerable flexibility in the apparatus for carrying out the methods of the taggants identification. As a general matter, the excitation source may be any inexpensive near-infrared laser diode and the detector may be any convenient CCD array or matrix. The laser light is preferably delivered with the aid of a fiber optic probe to a small area covered with a mark incorporating a Raman-active compound (or a mixture thereof) of the invention, and the Raman light emanating from that area is collected and directed by a fiber optic conduit of the probe to the detector. An electrical signal representing the intensity of light in the Raman emission bands provides a molecular fingerprint of the Raman-active compound(s) present. In a preferred embodiment, the process as set forth above is carried out by a miniature or portable Raman spectrometer directly xe2x80x9cin the fieldxe2x80x9d.